1. Field of the Invention
The present invention relates to an organometallic complex. In particular, the present invention relates to an organometallic complex that is capable of converting a triplet excited state into luminescence.
2. Description of the Related Art
In recent years, there has been an active development of light-emitting elements in each of which an organic or inorganic compound having a light-emitting property is used as a light-emitting substance. In particular, a light-emitting element called an EL (electroluminescence) element has attracted attention as a next-generation flat panel display element because it has a simple structure, in which a light-emitting layer containing a light-emitting substance is provided between electrodes, and characteristics, such as feasibility of being thinner and more lightweight and responsive to input signals and capability of driving with direct current at low voltage. Moreover, a display using such a light-emitting element has high contrast, excellent image qualities, and a wide viewing angle. Furthermore, such a light-emitting element is a planar light source, and accordingly its applications to light sources such as backlights of liquid crystal displays and lighting have been under contemplation.
In the case where the light-emitting substance is an organic compound having a light-emitting property, the emission mechanism of the light-emitting element is a carrier-injection type. Specifically, by application of a voltage to electrodes between which the light-emitting layer is interposed, electrons and holes injected from the electrodes recombine to raise the light-emitting substance to an excited state, and light is emitted when the substance in the excited state returns to the ground state. Possible excited states are a singlet excited state (S*) and a triplet excited state (T*). In addition, the ratio of S* to T* formed in the light-emitting element is statistically considered to be 1:3.
In general, the ground state of an organic compound having a light-emitting property is a singlet state. Light emission from a singlet excited state (S*), which is electron transition between the same multiplicities, is referred to as fluorescence, and light emission from a triplet excited state (T*), which is electron transition between different multiplicities, is referred to as phosphorescence. At room temperature, observations of a compound which emits fluorescence (hereinafter referred to as a fluorescent compound) usually show only fluorescence without phosphorescence. Hence, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on a S*-to-T* ratio of 1:3.
On the other hand, use of a phosphorescent compound can increase the internal quantum efficiency to 75% to 100% in theory. In other words, an element using a phosphorescent compound can have three to four times as high emission efficiency as that of an element using a fluorescent compound. For these reasons, a light-emitting element using a phosphorescent compound has been actively developed in recent years in order to achieve a highly-efficient light-emitting element (e.g., see Non-Patent Document 1). As the phosphorescent compound, organometallic complexes that have iridium or the like as a central metal have particularly attracted attention because of their high phosphorescence quantum yield.
Further, a light-emitting element using a phosphorescent compound is disclosed which uses a light-emitting layer containing an organic low molecular hole-transport substance and an organic low molecular electron-transport substance as host substances and the phosphorescent compound as a dopant and has improved lifetime and efficiency (see Patent Document 1).
An advantage of use of the highly-efficient light-emitting element is that power consumption of an electronic device using the light-emitting element can be reduced, for example. With recent attention to the energy problems, power consumption is becoming a major factor controlling the trends in consumer purchases and thus attains considerable importance.